Our long-term goal is to identify the mechanisms that govern metastatic growth and dormancy. Due to the morbidity and mortality associated with metastatic disease, finding ways to stop metastatic growth either by inducing dormancy or even eradicating disseminated dormant cells, would greatly reduce deaths caused by cancer. We previously showed that dormant cancer cells have highly activated p38 which blocks ERK activation generating a balance that favors the maintenance of dormancy. Based on this observation we initiated a proteomic analysis of down-stream targets of p38 signaling. This search yielded two major findings, one that linked integrin signaling and p38 activation to the activation of two endoplasmic reticulum receptors, PERK and IRE1alpha, which induced an integrated stress response (ISR) and a second that indicated an altered sub-cellular localization of hnRNPAl, a change that affects mRNA splicing and/or stability. We now propose 3 specific aims: In Specific Aim 1 we will determine the causal link between integrin signaling, p38 activation and the activation of PERK and !RE1cc as inducers of an ISR in dormant cells. We will test our hypothesis that activation of this pathway can force malignant cells into a state of protracted dormancy. We will inhibit or activate components of the integrin->p38->PERK/IRE1alpha pathways using genetic or pharmacological approaches and test their effect on metastatic growth using the xenograft nude mice model. In Specific Aim 2, we will investigate the link between high p38 activity and localization of hnRNPAl to the cytosol in dormant cells as a mechanism that regulates post-transcriptional gene expression of ISR regulated genes and 8 tumor suppressors genes that we have found, are induced by p38 in dormant cells. Using a combination of immunoprecipitation, RT-PCR and gene arrays (ribonomics) we will identify the mRNAs regulated by the ISR and p38 that are associated with hnRNPAl in high p38 (dormant) and low p38 (proliferative) cells. In Specific Aim 3 we will use a novel approach in which we tag ERK or p38 signaling pathways with GFP. In so doing we are able to monitor the state of ERK or p38 activation in primary and metastatic cancer cells in vivo. We will use tumorigenic and dormant cells engineered to express an ERK- or p38-induced GFP to isolate from primary tumors, cells that activate p38 as they become dormant. These cells will be used to obtain total mRNA expression profiles. These will be linked to p38 signaling and will provide a dormancy gene expression signature. We will use this dormancy signature to test if it is present in proliferating metastasis (high ERK/low p38) in nude mice and determine if it informs on their behavior. Our proposal will reveal the functional contribution of two novel pathways to tumor dormancy: one linking integrin, p38 and ER-stress signaling in dormant cells and a second one where p38 regulation of hnRNPAl regulates post-transcriptional gene expression of ISR regulated genes and known tumor suppressors. Finally, our third aim will provide the first p38-regulated gene expression signature of dormant cells and will teach us if this signature is lost in growing metastasis These results may provide avenues for designing curative and/or life prolonging therapies for cancer.